Using Multi-Value - Wasmtime

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# Using multi-value

You can also browse this source code online and clone the wasmtime repository to run the example locally.

This example shows off how to interact with a wasm module that uses multi-value exports and imports.

## `multi.wat`

    (module
      (func $f (import "" "f") (param i32 i64) (result i64 i32))

      (func $g (export "g") (param i32 i64) (result i64 i32)
        (call $f (local.get 0) (local.get 1))
      )

      (func $round_trip_many
        (export "round_trip_many")
        (param i64 i64 i64 i64 i64 i64 i64 i64 i64 i64)
        (result i64 i64 i64 i64 i64 i64 i64 i64 i64 i64)

        local.get 0
        local.get 1
        local.get 2
        local.get 3
        local.get 4
        local.get 5
        local.get 6
        local.get 7
        local.get 8
        local.get 9)
    )

## `multi.c`

    /*
    Example of instantiating of the WebAssembly module and invoking its exported
    function.

    You can build using cmake:

    mkdir build && cd build && cmake .. && cmake --build . --target wasmtime-multi

    Also note that this example was taken from
    https://github.com/WebAssembly/wasm-c-api/blob/master/example/multi.c
    originally
    */

    #include <inttypes.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <wasm.h>
    #include <wasmtime.h>

    static void exit_with_error(const char *message, wasmtime_error_t *error,
                                wasm_trap_t *trap);

    // A function to be called from Wasm code.
    wasm_trap_t *callback(void *env, wasmtime_caller_t *caller,
                          const wasmtime_val_t *args, size_t nargs,
                          wasmtime_val_t *results, size_t nresults) {
      printf("Calling back...\n");
      printf("> %" PRIu32 " %" PRIu64 "\n", args[0].of.i32, args[1].of.i64);
      printf("\n");

      results[0] = args[1];
      results[1] = args[0];
      return NULL;
    }

    // A function closure.
    wasm_trap_t *closure_callback(void *env, wasmtime_caller_t *caller,
                                  const wasmtime_val_t *args, size_t nargs,
                                  wasmtime_val_t *results, size_t nresults) {
      int i = *(int *)env;
      printf("Calling back closure...\n");
      printf("> %d\n", i);

      results[0].kind = WASMTIME_I32;
      results[0].of.i32 = (int32_t)i;
      return NULL;
    }

    int main(int argc, const char *argv[]) {
      // Initialize.
      printf("Initializing...\n");
      wasm_engine_t *engine = wasm_engine_new();
      wasmtime_store_t *store = wasmtime_store_new(engine, NULL, NULL);
      wasmtime_context_t *context = wasmtime_store_context(store);

      // Load our input file to parse it next
      FILE *file = fopen("examples/multi.wat", "r");
      if (!file) {
        printf("> Error loading file!\n");
        return 1;
      }
      fseek(file, 0L, SEEK_END);
      size_t file_size = ftell(file);
      fseek(file, 0L, SEEK_SET);
      wasm_byte_vec_t wat;
      wasm_byte_vec_new_uninitialized(&wat, file_size);
      if (fread(wat.data, file_size, 1, file) != 1) {
        printf("> Error loading module!\n");
        return 1;
      }
      fclose(file);

      // Parse the wat into the binary wasm format
      wasm_byte_vec_t binary;
      wasmtime_error_t *error = wasmtime_wat2wasm(wat.data, wat.size, &binary);
      if (error != NULL)
        exit_with_error("failed to parse wat", error, NULL);
      wasm_byte_vec_delete(&wat);

      // Compile.
      printf("Compiling module...\n");
      wasmtime_module_t *module = NULL;
      error =
          wasmtime_module_new(engine, (uint8_t *)binary.data, binary.size, &module);
      if (error)
        exit_with_error("failed to compile module", error, NULL);
      wasm_byte_vec_delete(&binary);

      // Create external print functions.
      printf("Creating callback...\n");
      wasm_functype_t *callback_type =
          wasm_functype_new_2_2(wasm_valtype_new_i32(), wasm_valtype_new_i64(),
                                wasm_valtype_new_i64(), wasm_valtype_new_i32());
      wasmtime_func_t callback_func;
      wasmtime_func_new(context, callback_type, callback, NULL, NULL,
                        &callback_func);
      wasm_functype_delete(callback_type);

      // Instantiate.
      printf("Instantiating module...\n");
      wasmtime_extern_t imports[1];
      imports[0].kind = WASMTIME_EXTERN_FUNC;
      imports[0].of.func = callback_func;
      wasmtime_instance_t instance;
      wasm_trap_t *trap = NULL;
      error = wasmtime_instance_new(context, module, imports, 1, &instance, &trap);
      if (error != NULL || trap != NULL)
        exit_with_error("failed to instantiate", error, trap);
      wasmtime_module_delete(module);

      // Extract export.
      printf("Extracting export...\n");
      wasmtime_extern_t run;
      bool ok = wasmtime_instance_export_get(context, &instance, "g", 1, &run);
      assert(ok);
      assert(run.kind == WASMTIME_EXTERN_FUNC);

      // Call.
      printf("Calling export...\n");
      wasmtime_val_t args[2];
      args[0].kind = WASMTIME_I32;
      args[0].of.i32 = 1;
      args[1].kind = WASMTIME_I64;
      args[1].of.i64 = 2;
      wasmtime_val_t results[2];
      error = wasmtime_func_call(context, &run.of.func, args, 2, results, 2, &trap);
      if (error != NULL || trap != NULL)
        exit_with_error("failed to call run", error, trap);

      // Print result.
      printf("Printing result...\n");
      printf("> %" PRIu64 " %" PRIu32 "\n", results[0].of.i64, results[1].of.i32);

      assert(results[0].kind == WASMTIME_I64);
      assert(results[0].of.i64 == 2);
      assert(results[1].kind == WASMTIME_I32);
      assert(results[1].of.i32 == 1);

      // Shut down.
      printf("Shutting down...\n");
      wasmtime_store_delete(store);
      wasm_engine_delete(engine);

      // All done.
      printf("Done.\n");
      return 0;
    }

    static void exit_with_error(const char *message, wasmtime_error_t *error,
                                wasm_trap_t *trap) {
      fprintf(stderr, "error: %s\n", message);
      wasm_byte_vec_t error_message;
      if (error != NULL) {
        wasmtime_error_message(error, &error_message);
        wasmtime_error_delete(error);
      } else {
        wasm_trap_message(trap, &error_message);
        wasm_trap_delete(trap);
      }
      fprintf(stderr, "%.*s\n", (int)error_message.size, error_message.data);
      wasm_byte_vec_delete(&error_message);
      exit(1);
    }